Field of Invention
The present invention relates to a field of micro-electro-mechanical systems (MEMS), in particular to a variable area capacitive lateral acceleration sensor and a preparation method thereof.
Description of Related Arts
Micro-electro-mechanical systems integrate a plurality of multiple micro mechanism, micro sensors, micro actuators, signal processing, control circuits, communication interfaces, and power supply. In the field of MEMS, excepting for pressure sensors, an acceleration sensor is the sensor with most studied, largest production and largest sales. A micro acceleration sensor is an important inertial sensor, and is used as a test device by converting the physical signal of acceleration into an electrical signal to facilitate the measurement. MEMS acceleration sensor has small size, light weight, little power consumption, fast start, low cost, high reliability, easy realization of digitization and intelligentization. Moreover, since the micromechanical structure has accurate process, highly reproducible, easy in integration, suits for mass production, the micro acceleration sensor is of high performance to price ratio.
According to the type of sensitive signals, MEMS acceleration sensors may be divided into capacitive type, piezoresistive type, piezoelectric type, thermoelectric coupling type, electromagnetic type and the like. Therein, the capacitive acceleration sensor has the advantage of high sensitivity, well stability, small temperature drift, etc, and is one of the most popular acceleration sensors. The capacitive acceleration sensor is provided with a fixed electrode and a movable electrode positioned in a seismic mass, after the capacitive acceleration sensor receiving an acceleration signal, the seismic mass is subjected to inertia force and moves in the opposite direction. The displacement of the seismic mass is limited by a spring and a damper. When the external acceleration is constant, the seismic mass has a certain displacement. As the external acceleration is changed, the displacement of the seismic mass changes accordingly. On the other hand, when the displacement of the seismic mass changes, the capacitance between the movable electrode plate that is connected to a sensitive seismic mass and the electrode plate that is fixed to an anchor zone changes accordingly; if the change of the output voltage of the sensor is measured, the displacement of the seismic mass is equivalent to be measured. Since the displacement of a seismic mass has one-to-one correspondence with the acceleration to be measured, then the output voltage and the external acceleration have a certain relationship, that is the external acceleration can be measured through the output voltage.
Capacitive MEMS acceleration sensors have various kinds of structure, mainly including sandwich structure and comb-finger structure. Sandwich capacitive acceleration sensors are basically variable gap capacitive type, and are mainly sensitive to longitudinal acceleration signal, because the seismic mass in the sandwich capacitive acceleration sensor can be made bigger, hence the sandwich capacitive acceleration sensor has high sensitivity and high test precision; while comb-finger capacitive acceleration sensors are basically variable area capacitive type, and are mainly sensitive to lateral acceleration signal, due to the employment of variable area pattern, the comb-finger capacitive acceleration sensors as prepared have better nonlinearity.
The method for preparing the sandwich capacitive acceleration sensors is mainly the method of bulk silicon micro-machining. In order to improve the detection sensitivity of micro acceleration sensors, various research institutions and companies have provided a variety of methods. Specifically, the methods include increasing the mass of a sensitive seismic mass and using a full symmetrical differential structure.
For example, W. S. Henrion et al. employ a double layer bonded silicon beam method to form a beam-mass structure whose double sides are parallel and symmetrical (cf. Sensors structure with L-shaped spring legs, U.S. Pat. No. 5,652,384), the process thereof may employ a method combining KOH etching with dry deep etching releasing. Namely: firstly, KOH is used to etch the silicon wafer from the back to a thickness of the remaining beam, dry deep etching is then used to release the beam-mass structure from the front; two same beam-mass are further bonded back to back to achieve a dual-side structure. The process is complicated and the cost is comparatively high.
The method for preparing the comb-finger capacitive acceleration sensors is mainly the method of surface micro-machining.
For example, ADXLXX series micro acceleration sensors of AD company in America use the capacitive acceleration sensor with uniform comb-fingers. The process for those sensors has good compatibility with the integrated circuit processing technology, however because of the structural relationships, the sensitive seismic mass are unable to make big, hence the sensors may not achieve high performance.